1. Field of the Invention
The present invention relates to apparatus which provides precise linear motion. More particularly, the invention relates to apparatus for use in conjunction with either plano or spherical interferometers for the purpose of rapidly and accurately measuring the distortion of either surfaces or transmitted wavefronts.
2. The Prior Art
The use of interferometry to measure optical components and systems has grown significantly due to technological advances in lasers, photosensors, and microcomputers. At the same time, relatively low-cost instruments have become more widely available for automatic data analysis and quantitative evaluation of interference patterns; see, for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Purchasing Directory, Book 2, pp. E-80-E-82 (1983).
Phase measuring interferometry (PMI) provides the optical path difference between the two wavefronts of the interferometer at each resolution element of the detector while phase modulating the interference pattern; see, for example, J. H. Burning, et al., "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses," Applied Optics, vol. 13, pp. 2693-2703 (1974); Gallagher, et al., U.S. Pat. No. 3,694,088 issued Sept. 26, 1972; N. Balasubramanian, U.S. Pat. No. 4,225,240 issued Sept. 30, 1980; M. Schaham, Proceedings SPIE, vol. 306, pp. 183-191 (1981); and H. Z. Hu, "Polarization heterodyne interferometry using a simple rotating analyzer. 1: Theory and error analysis," Applied Optics, vol. 22, pp. 2052-2056 (1983).
Phase measuring interferometry is capable of providing high data density and is insensitive not only to the intensity profile of the beam but also to the geometrical distortion in the optics or detector to first order. This makes phase measuring interferometry potentially more accurate than fringe pattern interferometry. It also enables the measurement of wavefronts of any fringe geometry and complexity as long as the maximum fringe density does not exceed one fringe/two resolution elements (pixels) of the detector.
In prior-art measuring techniques, the optical path difference, or phase, between the two wavefronts of the interferometer is altered, or modulated, by a known amount by one of the following means: (1) mechanically moving an optical element of the interferometer with a piezoelectric transducer, (2) rotating a phase retardation plate in the interferometer, (3) use of either an acousto-optic, electro-optic, or similar device in the interferometer, and (4) variation of the incident angle, see for example, Moore, U.S. Pat. No. 4,325,637 issued Apr. 20, 1982. Most of the prior-art phase modulators require the use of refractive optics in the measurement leg of the interferometer for either large aperture or fast spherical measurements. The refractive optics are not only a serious source of error but also quite expensive. Similarly, the use of phase retardation elements for large apertures and fast spherical measurements presents the same problems.
Prior-art phase modulators of the mechanical type have been used only with small aperture diameter, i.e., 2-10 cm, plano optical elements because of their limited load carrying capacity and because of the errors introduced in fast spherical measurements due to the imprecise motion. A phase modulator of the mechanical type must be capable of moving an optical element in an extremely precise, tilt-free translation in a straight line over a distance of one-half the wavelength of the radiant energy of the interferometer. A practical phase modulator of the mechanical type must also be capable of supporting a variety of reference surfaces and operating at any orientation relative to the vertical so that it can be used for many applications. Furthermore, when an optical element with a non-plano surface must be moved to achieve the phase modulation, elaborate corrections must be made in the data analysis, see for example, R. C. Moore, "Direct measurement of phase in a sphericalwave Fizeau interferometer," Applied Optics, vol. 19, pp. 2196-2200 (1980). Variation of the incident angle is useful for low precision, plano measurements. Specifically, all of the prior-art modulation techniques are expensive and introduce significant measurement errors when large aperture or fast spherical wavefront measurements are required.
In a copending application, Ser. No. 515,393, filed July 20, 1983 "Interferometric Wavefront Measurement", assigned to the assignee of this application, an improved phase modulation apparatus and method is disclosed; however, it can be used only at wavelengths for which diode lasers with sufficient power and coherence length are available, and it cannot be used in an equal path interferometer.
While prior-art modulation techniques are useful for some applications, it is desirable to do phase measuring interferometry with a modulation technique which is not subject to the limitations inherent in the prior-art modulation techniques.